1. Field of Invention:
This invention relates generally to masonry anchors, and more particularly to a self-tapping, screw-type masonry anchor which when installed in a hole bored in a masonry structure to secure a fixture or other object thereto, is then highly resistant to forces which seek to pull the anchor out of the hole.
2. Status of Prior Art:
It is frequently necessary to secure fixtures, brackets, channel pieces and other more or less heavy objects to the surface of a masonry structure formed of concrete, brickwork or other masonry material. These objects are attached to the masonry structure by masonry anchors.
Concrete is made by mixing cement and an aggregate of inert particles of varying size, such as a combination of sand or broken stone screenings with gravel. Compressive strength is generally accepted as the principal index to the structural quality of concrete. Mixtures for concrete masonry structures, such as walls and partitions, ordinarily employ aggregates having a maximum size of one half inch.
Masonry brick is usually formed from clay, shale or pumice hardened by heat. Bricks for this purpose are available in different degrees of hardness, depending on the material used in making the brick.
But regardless of the material used in forming a masonry structure, when a hole is drilled therein to receive a metal masonry anchor whose shank frictionally engages the wall of the hole drilled in the masonry, because the wall of the hole has abrasive characteristics, it will abrade and wear the shank to a degree that depends on the physical properties of the anchor metal. Thus a shank made of tungsten carbide metal which has an exceptionally high degree of Rockwell A hardness will experience little wear, whereas a shank made of stainless steel, which is not nearly as hard, is subject to much greater wear.
The patent to Ernst, U.S. Pat. No. 3,937,119 discloses a self-tapping, screw-type metal anchor which has a sharp-crested helical male thread surrounding the shank of the anchor. The convolutions of the thread running the length of the shank. This male thread, when the anchor is turned into a hole drilled in masonry, functions to tap the wall of the hole to create an internal female thread in the wall. Since the male thread on the shank mates with the female wall thread, the anchor is then resistant to pull-out forces which seek to back the anchor out of the hole.
Also included in the Ernst anchor is a second thread whose convolutions surround the shank in the successive spaces between the convolutions of the cutting thread. The crests of the second thread have a diameter which is smaller than that of the cutting thread crests and about equal to the diameter of the masonry hole. This second thread acts as a guide thread to center the anchor in the masonry hole, so that the anchor is not permitted to tilt as it is turned into the hole.
As explained in the Ernst patent, the guide thread, by preventing tilting of the anchor in the masonry hole, enhances its pull-out resistance; for if the anchor were tilted, the crests of the cutting thread would then not be properly embedded in the hole.
In a preferred form of the Ernst masonry anchor, the crests of the cutting thread are not of uniform diameter throughout the length of the shank, but decrease progressively from the trailing end of the shank toward the tip or leading end which is inserted into the mouth of the hole. Because the cutting thread is tapered, according to Ernst, it can be turned into the hole with a minimal amount of torque by a conventional threaded fastener tool, such as a screwdriver.
Another feature of the Ernst masonry anchor resides in a series of notches formed on the crests of the cutting thread along the full length of the shank. These notches, in conjunction with the tapering of the crest diameters of the cutting thread, make it possible to insert the anchor in the masonry hole without unduly disturbing the wall of the hole. Masonry material tends to crumble when worked. But with the gradual chipping carried out by the notches during continued rotation of the anchor, this serves to ensure the firm embedment in the wall of at least the relatively large diameter crests at the trailing end of the cutting thread.
According to Ernst, as aggregate particles of the masonry hole wall are gradually removed from the wall by the action of the notches, the material so removed is trapped in the spaces between the convolutions of the cutting thread. The retention of these particles in the spaces enhances the pull-out strength of the anchor.
We have found, however, that an anchor of the Ernst type actually offers low resistance to back out resulting from vibratory and other external forces transmitted to the installed anchor through the masonry structure. These forces may originate from operating machinery mounted on the masonry structure or heavy moving trucks or other vehicles traveling near the site of the masonry structure. Also, high winds are a factor when the masonry structure is exposed thereto. These forces seek to back the installed anchor out of the masonry hole.
The reason the Ernst masonry anchor, which has a tapered male cutting thread, has poor back-out resistance, is that when it is turned into a masonry hole and taps the wall of this hole, it then creates on this wall an tapered internal female thread that mates with the tapered male thread.
When a male tapered element is fully socketed within a complementary female tapered element, these elements are then contiguous. But if the male element is axially withdrawn even slightly, then the tapered male element is separated from the complementary female element.
An installed Ernst anchor having a tapered male cutting thread on the shank is then intermeshed with the tapered internal female thread in the wall of the masonry hole. But if this anchor, which serves to hold an object against the masonry surface, is subjected to vibratory forces that act to back the anchor out of the hole even to a slight degree, when this happens, the male thread disengages from the female thread. The anchor is then loose and is free to pull out of the hole, and it ceases, therefore, to hold the object against the masonry.
In the embodiment of the Ernst anchor in which the cutting thread is of uniform diameter and therefore has no taper, as this anchor is turned into a hole bored in masonry, the crests of the leading end of the shank which have to work their way almost the full depth of the hole will experience the greatest amount of wear; hence this cutting thread will have a taper imparted thereto in the course of being turned in. But in the Ernst embodiment in which the cutting thread is tapered, since the crests at the leading end of the shank have the smallest diameter and those at the trailing end the greatest diameter, the amount of wear to which these crests are subjected in the course of turning in the anchor is about the same; hence the taper will be retained. In either case, the installed anchor will have poor resistance to being backed out of the hole by vibratory forces.
Another drawback of the Ernst screw-type masonry anchor whose cutting thread has a tapered formation is that it requires a high degree of torque to drive the anchor into the masonry hole.
The reason for this is that the diameter of the cutting crests increases progressively from the leading to the trailing end of the shank. Hence as the anchor is rotated to tap its way into the masonry hole, the wall of the hole is engaged by crests of increasing diameter, this resulting in a progressive rise in drag and greater resistance to rotation.
The high torque required to turn the Ernst masonry anchor into the masonry hole may limit the depth of entry, for a point is reached where even the high torque is insufficient to overcome the resistance to turning, and further advance of the anchor into the hole is arrested. And when the anchor is so arrested, the continued application of a high torque may cause the anchor to fracture or snap in half.